1. Field of the Invention
The present invention relates to a communication system and a node for the communication system, the communication system including a trunk line to transmit differential signals and branch lines each branching from the trunk line and connected with the node, the node being configured to perform communication with other nodes by the differential signals.
2. Description of Related Art
The number of ECUs (Electronic Control Units) mounted on a vehicle and the amount of data transmitted between the ECUs through a control system are increasing steadily with the controls performed in the vehicle becoming complicated and sophisticated. Accordingly, there is a growing demand for a high performance communication system.
There is known a vehicle-mounted communication system having a bus structure including a trunk line constituted of a pair of signal lines for transmitting differential signals, and a plurality of branch lines each branching from the trunk line and connected with an ECU. The differential signals have two signal levels, a dominant level and a recessive level, in order to transmit data as binary signals by associating the dominant level and a recessive level respectively with the logical value 0 and the logical value 1, for example.
However, such a communication system has a problem in that there occurs a signal reflection in the direction from the branch line to the trunk line at the branch point therebetween when the signal level is changed. The intensity of the signal reflection increases as the length of the branch line increases, and also as the communication speed of the communication system increases. When the signal reflection is excessively large, the differential signals are distorted to a large extent as shown in FIG. 10, causing communication accuracy to degrade, and also causing communication errors. The signal reflection occurs due to change of the characteristic impedance of the signal lines constituting the branch line caused when the length of the signal lines is changed, or when the number of the mounted ECUs is changed, for example. That is, the signal reflection occurs due to a characteristic impedance mismatch.
To solve this problem, it is known to insert a resistive element to each branch line for impedance matching to suppress reflection therefrom. For example, refer to Japanese Patent Application Laid-open No. 7-202947 or 2006-237763.
However, such a conventional technique has a problem in that signal amplitudes may be lowered to such an extent that the communication cannot be reliably performed depending on the inserting position of the resistive element. More specifically, in each of the communication systems disclosed in the above patent documents, since a resistive element is connected in series to the signal line, signal amplitudes are lowered considerably. Furthermore, in each of the communication systems disclosed in the above patent documents, since the resistive element has to be inserted into a connector in order to branch the branch line from the trunk line, there are disadvantages in manufacturing ease and cost. More specifically, since it is necessary to insert an additional circuit to the signal line, labor and cost to manufacture the communication system increase.